-preview=in might break code

[Adam D. Ruppe]

On Saturday, 3 October 2020 at 14:10:34 UTC, Steven Schveighoffer 
wrote:
> What happens is, I write code that assumes some `in` parameter 
> won't change

That code is kinda already buggy since in just means you won't 
change it, but somebody else might.

I know it is weird when looking at something that is typically a 
value copy, but in is still based on const, not immutable, so you 
must keep some expectation that it might be changed by someone 
else. (BTW speaking of ref optimizations, any immutable could 
prolly be passed by reference implicitly as well.....)

But maybe like you said later, the spec should say any `in` is 
treated at the language level as a `ref` (except for rvalue 
issues of course) just optimized to value in defined ABI places. 
That'd probably be good enough.